1. Field of the Invention
The present invention relates to a system for detecting an on-hook/off-hook condition of portable terminal equipment with respect to a charger.
2. Description of the Related Art
An ultimate object of communication is, of course, to enable intention or information to be transmitted or exchanged at once anytime, from anywhere, and by anybody. In addition to a conventional intercommunication between fixed points, a mobile communication has been gradually developed. The mobile communication means a communication between a mobile body such as ship, automobile, or airplane and a fixed point such as home or office, and also means an intercommunication between two mobile bodies. In recent years, a portable telephone as a kind of means for carrying out the mobile communication has been extensively developed.
Since the portable telephone is used as a user is moving, it is essential to employ a battery pack as a power source for the portable telephone. Actually, various types of rechargeable battery pack are employed. In particular, a nickel-hydride battery pack (Ni-MH battery pack) has recently been remarked as a hopeful power source for portable terminal equipment including the portable telephone because of a large ratio of electric capacity/battery volume, which can make the equipment compact and extend an operation time. In the nickel-hydride battery pack, however, a circuit for stopping charging at the time of full charge and a safety circuit for detecting a battery temperature are indispensable for ensuring the safety. Accordingly, when the full charge is reached, a charging current is cut off by the charge stopping circuit, and it is therefore necessary to detect an on-hook/off-hook condition of the portable telephone by any method rather than monitoring of the charging current.
Conventionally, a nickel-cadmium battery pack (Ni--Cd battery pack) is used as a power source for the portable telephone. In the nickel-cadmium battery pack, a slight charging current is continued to flow even after the full charge, that is, a so-called trickle charge is allowed. Accordingly, the on-hook/off-hook condition of the portable telephone is detected by monitoring a charging current. Further, since the nickel-cadmium battery pack is primarily used as a rechargeable battery pack, there occur no problems regarding the mixing with other types of battery pack and the safety against a faulty battery pack or the like.
To further reduce the size and the weight of the portable terminal equipment and further extend the operation time, it has recently been considered to adopt the nickel-hydride battery pack. As mentioned above, the nickel-hydride battery pack must include a charge stopping circuit for stopping a charging current at the time of full charge and a safety circuit for detecting a temperature of the battery pack, so as to prevent the shortening of a life cycle due to continuation of charge even after the full charge.
In the nickel-hydride battery pack including the charge stopping circuit, the charging current is cut off at the time of full charge, so that the on-hook/off-hook condition of the portable terminal equipment cannot be detected by monitoring the charging current. Further, although the nickel-cadmium battery pack is a mainly used battery pack, and no consideration for the mixing with other types of battery pack need not be taken in particular, there is a problem regarding the safety because the outputs from charging terminals of a charger continue to be generated even when no battery pack is connected to the charger.